Loudspeakers generally comprise a frame, motor structure, a diaphragm, a lower suspension or spider and a surround. In one common type of speaker, the motor structure includes a top plate spaced from a back plate with a permanent magnet mounted therebetween. The magnet and top plate define an air gap within which a hollow, cylindrical-shaped voice coil is axially movable with respect to a fixed pole piece which is centrally mounted atop the back plate.
The voice coil generally comprises a cylindrical former which receives a winding of wire. The diaphragm extends between the voice coil and the surround, which, in turn, is mounted to the upper end of the frame. The spider is connected at one end to the voice coil, and at its opposite end to a point between the upper and lower ends of the frame. In this construction, one cavity or space is formed in the area between the diaphragm and spider, and a second cavity is formed in the area between the spider and the top plate of the motor structure. Many speaker designs include a dust cap mounted to the diaphragm in position to overlie and cover the voice coil and pole piece.
In the course of operation of a speaker of the type described above, electrical energy is supplied to the voice coil causing it to axially move relative to the pole piece and within the air gap formed by the top plate and magnet. The diaphragm, spider and the surround, move with the excursion of the voice coil. A pervasive problem associated with speaker operation involves the build up of heat produced by the voice coil and radiated to surrounding surfaces. Both the voice coil and top plate become quite hot during speaker operation which can reduce the power handling of the speaker, and increase power compression, i.e. a reduction in acoustic output due to temperature-related voice coil resistance.
A variety of designs have been employed in the prior art to address the problems associated with heat build up in speakers. Much of the design effort has been devoted to creating a flow of cooling air over the voice coil itself, such as disclosed, for example, in U.S. Pat. No. 5,042,072 to Button; U.S. Pat. No. 5,081,684 to House; and U.S. Pat. No. 5,357,586 to Nordschow et al. A typical construction in speaker designs of this type involves the formation of passages in or along the voice coil which form a flow path for the transfer of cooling air from the cavity between the voice coil and the dust cap and/or diaphragm, and vent openings usually formed in the back plate of the motor structure. An air flow through these passages is created in response to movement of the diaphragm with the excursion of the voice coil. When the diaphragm moves in one direction, air is drawn from outside of the speaker, through the vent opening in the back plate, along the passages in or along the voice coil and then into the cavity. Movement of the diaphragm in the opposite direction creates a flow out of the cavity along the reverse flow path.
One problem with the approach described above is that the design and construction of the flow passages often do little more than provide venting of the area or cavity between the diaphragm and voice coil. The actual air flow generated by movement of the diaphragm is typically relatively low volume. As a result, very little cooler ambient air from outside of the speaker actually flows along the voice coil to provide effective cooling. Additionally, little or no air flow is directed along the top plate, which remains hot.
Alternative designs depend upon thermal conduction and convection to cool the voice coil and/or top plate. Typically, structure associated with the frame is positioned in engagement with the top plate and proximate the voice coil of the motor to provide a heat sink or thermally conductive path along which heat can move from the relatively hot voice coil and top plate to the relatively cool frame. In U.S. Pat. No. 4,933,975 to Button, for example, a collar is positioned at the inside diameter of the frame near the voice coil, a base plate is connected to the collar and rests atop the top plate of the motor, and, a number of fins extend radially outwardly from the collar along the base plate to the outer portion of the frame. Collectively, these elements form a heat sink for the conduction of heat away from the voice coil and top plate, outwardly to the frame.
Constructions of the type described above provide some benefit, but reliance on conduction and convection alone to remove heat from the top plate and voice coil is of limited effectiveness with today's high performance, high excursion speakers. This is particularly true in applications such as vehicle speakers where space is at a premium and the speaker frame must be as compact and light weight as possible. In such designs, it is often not feasible to incorporate additional frame structure whose purpose is primarily or exclusively intended for the conduction of heat away from the voice coil and top plate.
At least one attempt has been made in the prior art to provide structure for the removal of heat from the voice coil using both conduction of heat into elements of the speaker frame, and the circulation of air past such elements. As disclosed in French Patent No. 2,667,212, a ring-shaped component is located between the bottom of the frame and the top plate of the motor which comprises a circular collar circumferentially disposed about the voice coil, and a number of vanes extending radially outwardly from the collar. The vanes are spaced from one another to define passages which are open at the outer edge of the frame, and open within the cavity formed between the spider or lower suspension of the speaker and the top plate of the motor.
The stated objective of the design disclosed in the French patent is to conduct heat away from the voice coil though the collar and vanes, and then create a flow of air over these surfaces resulting from the pumping action of the spider as it moves with the excursion of the voice coil during speaker operation. The problem with this approach is that the flow of air developed by the movement of the spider is essentially ineffective to transfer heat away from the frame and the vanes. Because the passages between adjacent vanes are open at their opposite ends, and open or exposed along their entire surface area within the cavity, any movement of air located in the area between the spider and top plate of the motor is at comparatively low velocity, and, hence, low volume. Little or no pressurization of the air is present within the relatively large volume cavity with the vanes completely exposed, and therefore little or no velocity is obtained in the air flow created by movement of the spider. As a result, limited heat transfer occurs between the low velocity, low volume flow of air past the vanes and collar, and the cooling effect of the "pumping" action of the spider is minimal.